By removing an electron from an atom or molecule, one creates a "handle" which allows one to grab it and hold onto it using electric and/or magnetic fields. The ability to trap and manipulate atoms and molecules in this way has long been recognized to hold great scientific and technological potential, with applications ranging from quantum-controlled chemistry to precision tests of the basic theories of physics which underlie virtually all of modern technology. Trapped atoms (and their ionized counterparts) have been studied by many groups, but it is more difficult (although potentially more rewarding) to trap and probe the relatively fragile molecules. The mostly untapped potential of molecules relates to their ability to rotate and vibrate, internal degrees of freedom which are absent in atoms. However, before the full potential of trapped molecules can be realized, techniques must be developed to determine their degree of rotation and vibration without destroying the molecules in the process. Here, the supported research group proposes capturing camera images of trapped molecules in such a way that they only appear in the picture if they have specific well-defined rotational and vibrational energies. These camera images will provide a means to non-destructively determine the rotation and vibration of trapped molecules, with detection resolution achievable down to the single-molecule level. It is anticipated that the development of these techniques will eventually find application in the chemical industry and in the realization of more advanced quantum computers.
With prior NSF support, the supported research group at Northwestern University demonstrated a technique to control the quantum rotational state of trapped molecules using a single femtosecond laser to simultaneously optically pump from all thermally populated excited levels. This technique works for molecular ions with a special internal structure, those possessing a so-called diagonal electronic transition, allowing many photons to be scattered before vibrations are excited. These molecules can be roughly thought of as the alkali atoms of the molecule world. In the present work, fluorescence of trapped molecular ions will be imaged directly on a CCD camera, lighting up certain locations in a one-dimensional Coulomb crystal where the single molecular ion at that site is in the probed quantum state. These CCD images will thus provide molecular state readout with single-molecule resolution. The group will then use fluorescence state readout in order to observe electric-dipole mediated rotational entanglement between co-trapped polar molecular ions. Future extensions of the work proposed here could include scattering-free detection of single molecules using optical phase shifts, directly using heavy molecular ion fluorescence readout for parity-violation and time-reversal symmetry violation searches, implementations of conditional quantum gates, entangling molecular ions with external circuits, and studies of decoherence dynamics in ion traps.
